1. Field of the Invention
The invention presented herein relates broadly to a process using a dry developer for the development of an electrical potential pattern presented at a surface of a receptor having application in the fields of electrographics and electrophotography and particularly to a process using a dry one-component developer of magnetically attractable, electrically insulating toner particles of develop an electrical potential pattern presented at the surface of a receptor wherein electrical charging of the toner particles is accomplished by direct injection of electrical charge onto the insulating toner particles from an electrode in an electric field rather than by inductive, triboelectric or other electrostatic means.
2. Discussion of the Prior Art
Many electrographic copying processes in use today involve the creation of an electrical potential pattern or image on a surface of a suitable receptor. One method of forming such an electrical potential pattern involves placing a uniform charge on a photoconductive insulating surface provided by the receptor and dissipating the charge selectively by exposure to a pattern of light and shadow to be reproduced. Another method utilizes electrically conductive pins or styli to generate an image-wise pattern of electrostatic charge on the surface of a dielectric provided by the receptor. Whether formed by these or some other methods, the electrical potential pattern is generally developed, i.e., made visible, by deposition of toner particles provided by the developer on the receptor according to forces generated by such electrical potential patterns. The developed image may then be fixed in place or transferred to a final support material, such as paper, and fixed thereto to form a permanent record of the developed potential pattern.
Presently, several techniques which employ a developer having finely divided dry toner particles can be used to develop the latent electrical potential pattern. These techniques can be broadly classified according to whether the toner particles are controllably and effectively charged by triboelectric means, inductive means or electrostatic means.
The two most common development techniques which employ triboelectric means to charge toner particles are called cascade development and magnetic brush development. Each technique utilizes a two-component developer comprised of finely divided insulating particles, generally referred to as toner, and relatively coarser particles of another composition, generally referred to as carrier. When the fine toner particles are brought into rubbing contact with the relatively coarse carrier particles, the toner particles become triboelectrically charged to a polarity opposite to that of the carrier particles and, thus, cling to the surface of the carrier particles. In conventional cascade development, the toner-carrier mixture is poured or cascaded over the electrical potential pattern bearing member and the triboelectrically charged toner particles deposit preferentially in regions of the surface where there is a preponderance of charge of the opposite polarity. In magnetic brush development, magnetically attractable carrier particles are generally employed and a magnetic force is used to provide adherence of the toner-carrier mixture to a support member. The mixture is then presented to the image bearing surface to allow the triboelectrically charged toner particles to deposit in the regions of the surface where there is a preponderance of charge of the opposite polarity.
Development techniques which employ inductive means to charge the toner particles generally utilize a one-component developer of finely divided, dry, conductive toner particles. Such a technique is described in U.S. Pat. No. 3,909,258 to Kotz. In this case, the one-component developer of finely divided, electronically conductive toner particles are also magnetically attractable and are transported by a cylindrical support member that may be rotated and is spaced from the receptor. The toner particles are uniformly magnetically attracted toward the support member and are inductively charged via an electrically conductive path or "circuit" which includes the support member and the toner particles presented between the support member and the electrical potential pattern bearing surface of a receptor. A fixed direct current electrical potential or ground is generally connected in the circuit which with the potential pattern at the surface of the receptor establishes the electrical potential for producing the electronic current flow for inductively charging the toner particles. Deposition of the toner particles occurs in the image areas of the potential pattern bearing surface of the receptor when the magnitude of charge induced on the toner particles results in a transient electrical transfer force greater than and opposed to the magnetic force of attraction on the toner particles.
Development techniques which employ electrostatic means to charge the finely divided, dry toner particles generally utilize a one-component developer provided by toner particles of insulative material and rely on electrostatically generated ions sprayed on the toner particles to charge them. An apparatus for such a technique is described in U.S. Pat. No. 3,552,355 to Flint. In this case, a supply of one-component developer of magnetically attractable, electrically insulating toner particles is continuously moved around a transport roll and past a charging device whereupon ions that are electrostatically generated by means such as a corona are sprayed on the toner particles. The polarity of the electrostatically generated ions is generally opposite to that of the electrical potential pattern to be developed such that the charged toner particles will be preferentially deposited in these oppositely charged regions of the electrical potential pattern bearing surface. This apparatus also requires the presence of oscillating wave forming elements situated between the transport roll and the surface to be developed which functionally provides undulation of the toner particles in the development region.
While all of the above techniques have certain advantages in particular situations, each one suffers from disadvantages which impair their utility in actual machines.
In the conventional cascade development technique, the toner portion of the developer has a definite charge polarity that is not reversible without changing the toner portion and/or the carrier portion of the developer. Thus, positive and negative developed images cannot easily be made. Also, the developed images are hollow and solid areas are not filled in resulting in low fidelity development compared to the original charge pattern. The triboelectric properties of the toner, while necessary to development, cause severe problems. Uneven charging of the toner causes backgrounding and uneven forces between carrier and toner result in varying threshold levels for deposition from toner to toner. Also, since the toner retains its charge for long periods of time, some toner escapes the development region during cascading and enters other parts of the apparatus causing mechanical problems familiar to those skilled in the art. These problems, coupled with the inherent problems of using a two-component developer system where only one component is depleted, definitely limit the utility of such techniques.
Magnetic brush development, being a form of cascade development, suffers from some of the above-mentioned disadvantages although it overcomes others. It requires two-component type developers which have the concomitant problems mentioned above. Also, due to the extreme frictional forces created by the mechanical brushing action of this system, the carrier particles become contaminated more rapidly resulting in a degradation of their triboelectric properties and requiring the toner-carrier mixture to be periodically replaced. Furthermore, the triboelectric intraction of the toner particles with the surface of the receptor necessitates tailoring the triboelectric properties of the toner to both the carrier material and the specific receptor material, and consequently, generally requires a specially selected developer for each type of receptor material.
The technique described in U.S. Pat. No. 3,909,258 to Kotz using a one-component developer of relatively conductive and magnetically attractable toner particles avoids many of the disadvantages associated with the above-mentioned cascade type techniques. This technique is capable of producing excellent quality likenesses of a desired configuration on a recording medium. However, problems arise in situations where it is desirable to transfer the developed image from the receptor to another medium such as plain paper. Such image transfer is particularly difficult to accomplish and control by electrical or electrostatic transfer means because the conductive nature of the toner particles permits rapid charge interchange with paper surfaces causing the attractive transfer forces on conductive toner particles to be generally low and variable as a function of time. Even in situations where the electrical properties of the medium to which the image is to be transferred can be controlled, the transferred image will often be inferior to the developed image in that it will lack the image density and edge sharpness required for a commercially acceptable copy.
While the apparatus described in U.S. Pat. No. 3,552,355 to Flint is capable of high density toner particle deposition, there are inherent problems associated with controllably charging insulating toner particles with a corona or similar type ion generating device. Corona devices are subject to well-known problems such as contamination, especially by airborne toner particles, which will result in non-uniform ion emission along the length of the corona wires and, thus, make it difficult to control the amount of charge deposited on the toner particles. Lengthwise non-uniform ion emission is also a characteristic of negative corona sources and would present an additional problem when negatively charged toner particles are desired. Furthermore, continuous ion emission from the corona source coupled with the probability that individual toner particles will be moved past this source many times can result in a time dependent change in the charge density per toner particle. These and other corona associated problems would be apparent as streaks and time dependent density variations in the developed images and, consequently, would limit the usefulness of the process. The wave-forming elements required in the development region further complicates the apparatus and limits the degree to which the spacing between the transport roll and the photoreceptor can be reduced. At times, a small gap is desirable to obtain optimum performance from a particular development process.